A better preserved version of the obverse here. The reverse isn't an uncommon theme on Roman coinage, finishing off a fallen foe, but it's still a bit grim. As usual, the emperor is seen as a much larger person than his inferior enemy.

This, I think, is why sestertii are so popular: they have enough room for complex scenes like this, especially when you're done listing all of Trajan's titles, which he seems to have insisted on.

The metal is brass, called oricalchum, which ages a bit differently that bronze. This has been harshly cleaned at some point and shows signs of erosion, particularly on the obverse, before the emperor's face.